Wednesday, March 09, 2016

An international team of experts from Europe and China has performed the
first simulations of globular clusters with a million stars on the
high-performance GPU cluster of the Max Planck Computing and Data
Facility. These – up to now - largest and most realistic simulations can
not only reproduce observed properties of stars in globular clusters at
unprecedented detail but also shed light into the dark world of black
holes. The computer models produce high quality synthetic data
comparable to Hubble Space Telescope observations. They also predict
nuclear clusters of single and binary black holes. The recently detected
gravitational wave signal might have originated from a binary black
hole merger in the center of a globular cluster.

Globular clusters are truly enigmatic objects. They consist of
hundreds of thousands luminous stars and their remnants, which are
confined to a few tens of parsecs (up to 100 lightyears) – they are the
densest and oldest gravitationally bound stellar systems in the
Universe. Their central star densities can reach a million times the
stellar density near our Sun. About 150 globular clusters orbit the
Milky Way but more massive galaxies can have over 10,000 gravitationally
bound globular clusters. As their stars have mostly formed at the same
time but with different masses, globular clusters are ideal laboratories
for studies of stellar dynamics and stellar evolution.

The dynamical evolution of globular clusters, however, is very
complex. Unlike in galaxies, the stellar densities are so high that
stars can interact in close gravitational encounters or might even
physically collide with each other. Because of these interactions there
are more tightly bound binary stars than for normal galactic field
stars. Moreover, in a process called mass-segregation more massive stars
sink to the center of the system.

The evolution of a globular cluster as a whole is further complicated
by the life cycle of both individual and binary stars. In the early
phases, massive stars (with more than 8 solar masses) suffer significant
mass-loss in a stellar wind phase and end their lifetime in
core-collapse supernova explosions. The remnants of these long-gone
stars are neutron stars or black holes; the latter with masses in the
range of ten to fifty solar masses. They are invisible for normal
electromagnetic observations and, until recently, could only be detected
indirectly.

The light from globular clusters is dominated by just a few hundred
very bright red giant stars. Most of the other stars in the systems have
a much lower mass than our Sun and very low luminosity. This is why the
Hubble Space Telescope has been a preferred instrument to observe the
stellar population of globular clusters. Color-magnitude diagrams (CMD)
obtained by Hubble have superior quality compared to ground-based
instruments due to very low photometric errors (creating sharp
structures like the main sequence or giant or white dwarf branches) and
very high sensitivity. Hubble for the first time observed low-luminosity
white dwarf features and low mass main sequences in high quality.

It has been a long-standing challenge to follow the evolution of a
massive globular cluster with self-consistent numerical simulations. For
the first time a team led by international experts at MPA, the Chinese
Academy of Sciences and Peking University has carried out the – up to
now – most realistic simulations of the evolution of a globular cluster
with initially one million stars orbiting in the tidal field of the
Milky Way for about 12 billion years. The simulations carried out at the
Hydra Supercomputer at the Max Planck Computing and Data Facility (MPCDF) as part of the international DRAGON project set a new standard in globular cluster modeling.

They
have been possible after significant improvements of the simulation
software on the laohu supercomputer of the Center of Information and
Computing at National Astronomical Observatories, Chinese Academy of
Sciences. The code has excellent parallel performance using,
simultaneously, multi-node parallelization, OpenMP on the nodes and
general-purpose Kepler K20 graphic cards acceleration (GPGPUs) to
compute the gravitational forces between the stars. A typical DRAGON
star cluster simulation used 8 nodes of Hydra with 160 CPU cores and
about 32k GPU threads, for a consecutive computing time of the order of
one year (8000 wall-clock hours).

The evolution of the stellar population of a globular cluster can now
be followed in great detail through all its dynamical and stellar
evolution phases, including the loss of stars in the tidal field of the
Milky Way. The evolution of single and binary stars with a large range
of masses (0.08 -100 solar masses) are followed through their major
evolutionary phases (Fig. 2). The DRAGON simulations have also been used
to prepare synthetic color magnitude diagrams (CMD) as observed by
Hubble (Fig. 3).

In the DRAGON simulations the black holes –
remnants of massive stars with masses of ten to fifty solar masses –
form a dense nuclear cluster in the center of the system (Fig. 2, panel
with white background). In classical astronomy this black hole cluster
can only be observed indirectly by its gravitational influence on the
luminous – and observable – stars. A few dozen black holes form binaries
and lose energy by gravitational radiation, a process included in our
simulations.

Recently the LIGO collaboration has detected gravitational wave
emission from a binary black hole coalescence (black hole masses of 36
and 29 solar masses) at a distance of 410 Mpc (see press release of the MPG ).
Our DRAGON clusters produce such binary black hole mergers with similar
parameters; about ten events in each cluster. Therefore we expect that
more events will be observed in the coming months or years. A more
detailed prediction for gravitational wave events from our models is
under way. It depends not only on the internal evolution but also the
number and distribution of globular clusters in the Universe. However,
we predict that globular clusters – similar to our DRAGON clusters – are
a possible origin of the recently observed spectacular gravitational
wave event.

The now detected black hole merger event is probably only the tip of
the iceberg. The dynamical evolution of the central regions of the
simulated clusters is dominated by hundreds (if not thousands) of single
and binary stellar mass black holes. Future studies should examine
whether such clusters of stellar mass black holes exist in centers of
most globular clusters rather than the predicted intermediate mass black
holes.

Thorsten Naab, Long Wang, Rainer Spurzem and Riko Schadow for the DRAGON collaboration

The DRAGON project:

TheDRAGONproject
is a supercomputing initiative of NAOC/CAS and KIAA/PKU (Beijing), MPA
(Garching), CAMK (Warsaw) and IoA (Cambridge) to investigate the
evolution of globular clusters with GPU supported high-performance
simulations. The team consists of Rainer Spurzem (supported by the
Thousand Talents Program of People's Republic of China at National
Astronomical Observatories of China (NAOC), Beijing and University of
Heidelberg), Long Wang and Thijs (M.B.N.) Kouwenhoven (Kavli Institute
for Astronomy and Astrophysics, Peking University), Peter Berczik (NAOC
and Main Astronomical Observatory of National Academy of Sciences of
Ukraine in Kiev), Sverre Aarseth (Institute of Astronomy, Cambridge),
Mirek Giersz and Abbas Askar (Nicolaus Copernicus Astronomical Center,
Warsaw), Thorsten Naab and Riko Schadow (Max-Planck-Institute for
Astrophysics, Garching), and further students and collaborators.
Computations are performed at theMax Planck Computing and Data Facility; software development and preliminary simulations at the laohu cluster of NAOC.